This invention relates to a hydraulic circuit for supplying hydraulic fluid to a plurality of hydraulic drive motors found in heavy equipment, such as excavators, backhoes and the like.
In recent years, the use of hydraulic drive systems for controlling the various functions in heavy equipment, for example, vehicles having earthworking implements, such as excavators, has become more common. For example, quite recently, completely hydraulic systems have been developed for use with heavy duty vehicles such as excavators.
In such systems, the actuation of control valves that control the flow of fluid to the various drive motors is accomplished by hydraulic fluid. One example of such a system incorporates a main hydraulic circuit that includes one or more main pumps and a plurality of hydraulically actuated valves that control the flow of fluid between the pumps, a reservoir and hydraulic motors associated therewith. Typically, the valves are self-centering and are opened by a control circuit that is capable of supplying small amounts of fluid under pressure from a source to opposite ends of the valves for actuating the valves. The fluid flow in the control circuit is controlled by manually and electrically operated valves and acts as a "pilot system" for actuating the main control valves.
Such a system has a number of advantages, the primary one being that the function that is being performed can be accurately controlled. For example, utilizing the "pilot system" for actuating the main valves gives the operator the ability to introduce very small amounts of fluid to the hydraulic motors.
While such systems have found a remarkable degree of success, there do exist certain limitations capable of being eliminated. For example, in one embodiment of such a system, parallel hydraulic circuits are utilized to rotate the cab or upper structure relative to the vehicle and to raise and lower the main lift boom. The hydraulic circuit utilized for rotating the upper structure is also utilized to supplement the other circuit to hoist or lift the boom at a greater than normal rate of speed.
In such a configuration, since the pumps utilized in the system have only a limited capability, it has been conventional to automatically disable or deactivate the supplemental fast hoist system capability when rotation of the upper structure is initiated. While usually desirable, this automatic circuit deactivation sometimes is inconvenient and unnecessary, and, therefore, it would be desirable to be able to override automatic deactivation in order to allow simultaneous rotation of the upper structure or cab and rapid lifting or hoisting of the main boom.